Remove useless specialization thanks to is_convertible being more robust.

Eigen/src/Core/util/SymbolicIndex.h

diff --git a/Eigen/src/Core/util/SymbolicIndex.h b/Eigen/src/Core/util/SymbolicIndex.h
index 1ade6d6..b9fe733 100644
--- a/Eigen/src/Core/util/SymbolicIndex.h
+++ b/Eigen/src/Core/util/SymbolicIndex.h
@@ -191,13 +191,6 @@
   enum { value = internal::is_convertible<T,BaseExpr<T> >::value };
 };
 
-// Specialization for functions, because is_convertible fails in this case.
-// Useful in c++98/11 mode when testing is_symbolic<decltype(fix<N>)>
-template<typename T>
-struct is_symbolic<T (*)()> {
-  enum { value = false };
-};
-
 /** Represents the actual value of a symbol identified by its tag
   *
   * It is the return type of SymbolValue::operator=, and most of the time this is only way it is used.